a. Field of the Invention
The present invention relates to engineering materials in general and more specifically to the coating of bulk engineering material with corrosion, oxidation or wear resistant inorganic materials.
b. Background of the Invention
Coatings are needed for protecting engineering components against mechanical, tribological and corrosive/oxidative degradation. In particular, engineering ceramics, by virtue of their light weight, high strength and thermal stability, have great potential for many high temperature applications. However, several of the best and toughest candidate ceramics, e.g. SiC, Si.sub.3 N.sub.4, and WC, require oxide coatings to resist corrosion and oxidation at high temperatures (900.degree. C. and up). In addition, their application as load-bearing materials is presently limited because of the difficulty of polishing their brittle surface to provide the tough, smooth surface "skins" that bearing components must have.
Conventional film deposition methods can be used to apply protective coatings to ceramics and other engineering materials. Coatings such as Al.sub.2 O.sub.3 can improve resistance to oxidation and corrosion at high temperatures. Other classes of ceramic coatings such as the nitrides, e.g. TiN, carbides, e.g HfC, or borides, e.g., TiB.sub.2, can generate smooth as well as tough, hard-coating surfaces. Unfortunately these coatings do not adhere well to many surfaces, preventing their evaluation and widespread use.
Adhesion is a property of the coating-substrate interface. Although the adhesive strength of a coating does not rely exclusively on chemical bonding, strong atom-atom bonds at the interface promote good adhesion. Thus good adhesion occurs when the coating can "react" with the substrate to form a compound or alloy, and poor adhesion occurs when the coating and substrate are unreactive toward each other.
The failure of ceramic coatings such as TiN and Al.sub.2 O.sub.3 (and of engineering coatings in general) to adhere to many substrates can be understood by reviewing well-known thermochemical principles. These principles predict the chemical reactivity and compound formation between two multi-element solids, which, in turn, can be described graphically with the aid of phase diagrams. A schematic Ti-N-Si ternary phase diagram in FIG. 1 will be used as an example to discuss the difficulty of adhering TiN coatings to a Si.sub.3 N.sub.4 ceramic substrate. In general, if any proportion of elements or compounds in the diagram are reacted, three thermodynamically stable phases arise as final products. These "reaction products" will usually differ in type and proportion from the initial reactants. The only exception to this rule is where the constituents are elements or compounds that lie on a "tie line" (solid line), e.g. the line connecting TiN to Si.sub.3 N.sub.4. In this case, the final products are the same (in type and proportion) as the initial ones. Since TiN and Si.sub.3 N.sub.4 are connected by a tie line, thermodynamics predicts that the two compounds won't react, i.e., a mixture of the two compounds results in no new compound formation. Thus, the bonding of TiN coatings to Si.sub.3 N.sub.4 lacks assistance from a chemical driving force.